Nonexocytotic noradrenaline release induced by pharmacological agents or anoxia in human cardiac tissue

In acute myocardial ischemia, noradrenaline is released locally from sympathetic varicosities by a Ca2+-independent nonexocytotic release mechanism that is effectively suppressed by inhibitors of the neuronal noradrenaline carrier (uptake(1)). The purpose of the present study was to elucidate the significance of free axoplasmic amine concentration and disturbed neuronal sodium homeostasis for nonexocytotic noradrenaline release in the human heart by comparing the release induced by anoxia with that induced by reserpine, tyramine, or veratridine. The overflow of endogenous noradrenaline and dihydroxyphenylethyleneglycol was assessed in human atrial tissue incubated in calcium-free Krebs-Henseleit-solution to prevent interferences by exocytotic release. The overflow of dihydroxyphenylethyleneglycol served as indicator of the free axoplasmic noradrenaline concentration. When vesicular uptake was blocked by the reserpine-like agent Ro 4-1284, high dihydroxyphenylethyleneglycol overflow was observed without concomitant noradrenaline overflow. If, however, Ro 4-1284 was combined with sodium pump inhibition (by omission of extracellular potassium) or with alteration of the transmembrane sodium gradient (by lowering the extracellular sodium concentration), both dihydroxyphenylethyleneglycol and noradrenaline were released. The indirectly acting sympathomimetic tyramine induced a marked increase in noradrenaline overflow which was accompanied by overflow of high amounts of dihydroxyphenylethyleneglycol, indicating interference of the drug with both vesicular catecholamine transport and amine transport via uptake(1). Likewise, veratridine induced an overflow of noradrenaline (which was prevented by blockade of uptake(1)) and dihydroxyphenylethyl eneglycol indicating a reserpine-like action of the drug. A disturbed energy status of the sympathetic neuron induced by cyanide intoxication or anoxia caused noradrenaline overflow which was suppressed by uptake(1) blockade. Blockade of sodium channels by tetrodotoxin effectively reduced noradrenaline overflow during cyanide intoxication but not during anoxia. Anoxia-induced noradrenaline release, however, was markedly suppressed by inhibition of Na+/H+ exchange with ethylisopropylamiloride, indicating the Na+/H+ exchange as the predominant pathway for sodium entry into the sympathetic neuron during anoxia. The results demonstrate that disturbed neuronal sodium homoeostasis and impaired vesicular storage function are critical conditions, causing nonexocytotic noradrenaline release in anoxic human cardiac tissue.